The overall aim of the proposed research is to further analyze by serologic means that region on Chromosome 6 of man which includes and surrounds the major histocompatibility locus (HLA). Primary emphasis will be directed toward the definition of the B lymphocyte antigens within the postulated I region. The number of loci to which these specificities belong will be assayed in family studies. These specificities will be correlated with the D locus specificities to determine the degree to which these two sets are identical. Secondary emphasis will be given to the study of antigenic specificities of other subpopulations of lymphocytes. A search will be undertaken for human antisera which identify alleles for new loci on the B, T and other subpopulations of lymphocytes as well as for the recognized alleles of the A, B, C, and D loci. The lymphocyte cytotoxicity test will be the primary serologic assay for presence of antigen. The relationship of the various serologically defined antigens will be correlated with disease susceptibility and with the immune response to various antigens.